You may be missing some setup for the X server.
One of the last installation steps will offer to update your X configuration file. Either accept that offer, edit your X configuration file manually so that the NVIDIA X driver will be used, or run nvidia-xconfig
More info here: http://us.download.nvidia.com/XFree86/Linux-x86_64/550.90.07/README/index.html
I fixed this by deleting Windows.